A magnetic photo-printing method for obtaining a hard copy is generally referred to as magnetography. In the case of printers using the magnetography, a thermo-magnetic printer, which utilizes a thermo-magnetic recording method for heating a magnetic recording medium so as to reduce its coercive force and for forming a magnetic latent image by further applying a magnetic field, has been conventionally known.
Methods for forming magnetic latent images in this thermo-magnetic printer are classified into an in-plane recording method and a perpendicular recording method from the standpoint of directions of magnetization.
As illustrated in FIG. 29(a), the in-plane recording method refers to a method in which a magnetic latent image is formed with its magnetization direction parallel to the surface of the magnetic recording medium. Moreover, as illustrated in FIG. 29(b), the perpendicular recording method refers to a method in which a magnetic latent image is formed with its magnetization direction perpendicular to the surface of the magnetic recording medium.
With respect to the comparison between these two methods, Journal of Electrophotographic Society (page 14, vol. 24, No. 4, 1985) describes the priority of the in-plane magnetization method as follows:
In general, when recording is carried out on a magnetic recording medium, a reduction in the recording magnetic field is observed due to a diamagnetic field exerted on the recording region; however, in the case when the recording is made with a width of 10 to 200 .mu.m on a recording film with a thickness (of 1 to 5 .mu.m) that is normally formed, the in-plane recording method makes the diamagnetic field smaller as compared with the perpendicular recording method. Therefore, the in-plane recording method provides a greater recording magnetic field and is more advantageous in recording efficiency as compared with the perpendicular recording method.
Moreover, in a method wherein the magnetization of a magnetic recording medium is preliminarily aligned in one direction so as to be initialized and wherein a magnetic latent image is recorded by forming magnetized regions whose magnetization direction is reversed to the initialized direction, the in-plane recording method exerts a greater magnetic attracting force with respect to the magnetic toner as compared with the perpendicular recording method. For this reason, the in-plane magnetization method is more preferable than the perpendicular recording method in terms of the recording method for thermo-magnetic printers.
Here, with respect to the magnetic recording medium for use in the thermo-magnetic printers, the following characteristics are required:
1 a greater magnetic attracting force (for high contrast), PA1 2 a high coercive force (for stability of recorded information), PA1 3 a capability for recording a magnetic latent image with high resolution.
Further, for reduction in power consumption and high speeds of magnetic-latent-image recording, it is also required that there be a greater reduction in the coercive force upon receipt of a temperature rise.
Conventional magnetic recording media, used in magnetic printers of the in-plane recording method, have been disclosed in Japanese Laid-Open Patent Publication No. 161572/1981 (Tokukaishou 56-161572), Japanese Laid-Open Patent Publication No. 161573/1981 (Tokukaishou 56-161573), and Japanese Laid-Open Patent Publication No. 161574/1981 (Tokukaishou 56-161574). In each of the Patent Publications, a magnetic recording medium made of CrO.sub.2 is formed on a sheet-like base layer. Further, Japanese Laid-Open Patent Publication No. 71076/1986 (Tokukaishou 59-71076) discloses a magnetic recording medium that is formed by applying CrO.sub.2 onto a polyimide film together with a binder.
Moreover, Japanese Laid-Open Patent Publication No. 20077/1986 (Tokukaishou 61-20077) and Japanese Laid-Open Patent Publication No. 20078/1986 (Tokukaishou 61-20078) disclose a magnetic recording medium made of a material containing Fe.sub.n N. This Fe.sub.n N is dispersed into a high-polymer-resin binding agent such as polyurethane and polyimide, and is applied to a base substrate made of polyethyleneterephthalate, polyimide, etc. having a thickness of 50 to 100 .mu.m, so as to have a thickness of 5 to 30 .mu.m.
However, toxicity of Cr, contained in the CrO.sub.2, has been conventionally pointed out, and it is preferable to avoid the use of CrO.sub.2 from the view of the current environmental issues. Further, in fact, the current situation makes it more difficult to obtain CrO.sub.2.
Moreover, in the case of Fe.sub.n N materials, the Fe.sub.n N, which contains iron as its ingredient, is susceptible to oxidation. Therefore, the problem with thermo-magnetic printers that are exposed to air at high temperatures is that the performance tends to deteriorate when they are used continuously for a long time.
Furthermore, each of CrO.sub.2 and Fe.sub.n N is basically dispersed into a highpolymer resin resolved in a solvent, and applied onto a drum base, a film sheet, etc. so as to form a film; and this method has the following disadvantages: first of all, biased distribution of CrO.sub.2 or Fe.sub.n N in the resin tends to cause unevenness in the characteristics of the magnetic recording medium, that is, it is difficult to control the dispersal state. Further, in the case of recording with a thermal head, a severe adhesion is required between the thermal head and the magnetic recording medium in order to maintain a uniform, sufficient thermal input to the magnetic recording medium. However, in the evaporating process of the above-mentioned solvent, irregularities in thickness tend to occur in the magnetic recording medium unless the evaporation rate is precisely controlled with respect to locations. In addition, in this process, it is difficult to control the film thickness of the medium in a uniform manner over a large area.
In order to solve the above-mentioned problems, the application of a sputtering method, an electron beam evaporation method, or other methods, in the film formation has been proposed. However, CrO.sub.2 and Fe.sub.n N exhibit superior characteristics when they are used in a dispersed form in a highpolymer binder in a coated film. For this reason, in the case of the film formation using a sputtering method or an electron beam evaporation method, it is difficult to obtain the same level of characteristics as the coated film.
Moreover, with respect to materials for the magnetic recording medium, in addition to the materials described above, amorphous magnetic materials made of a rare-earth metal and a transition metal are also listed. Japanese Laid-Open Patent Publication No. 100767/1987 (Tokukaishou 62-100767), Japanese Laid-Open Patent Publication No. 142361/1988 (Tokukaishou 63-142361) and Japanese Laid-Open Patent Publication No. 46766/1989 (Tokukaishou 64-46766) disclose amorphous magnetic materials made of NdDyFeCo to which are added elements, such as Ti, Al, Cu and Cr. In addition, Japanese Laid-Open Patent Publication No. 136673/1982 (Tokukaishou 57-136673) discloses amorphous magnetic materials made of rare-earth metals and transition metals, such as GdCo, TbFe, GdFe, DyTb, (GdTb)Fe, (GdTb)Co, TbFeO.sub.3, HoCo, DyFe and GdCoMo.
However, the above-mentioned amorphous magnetic materials are used for the perpendicular recording method, resulting in a problem in which they cannot be applied to the in-plane recording method as they are.
Even in the case of the above-mentioned amorphous magnetic materials, if the composition ratio of its rare-earth metal is made smaller, it is possible to manufacture an in-plane magnetization film that is applicable to the in-plane recording method; however, in order to obtain a sufficient attracting force to the magnetic toner by using the in-plane magnetization film that was manufactured by an amorphous magnetic material made of a rare-earth metal and a transition metal, it is necessary to make the thickness of the magnetic material greater.
In this case, if a thick in-plane magnetization film consisting of an amorphous magnetic material made of a rare-earth metal and a transition metal is manufactured by the sputtering method or the electron beam evaporation method, and applied to the magnetic recording medium of a magnetic printer, the height of serration in the serrate magnetization transition structure appearing in the border of magnetization inversion in a magnetic latent image formed on the magnetic recording medium tends to become greater. The resulting problem is that it is not possible to increase the resolution of the magnetic latent image recording (Electronic Information Communication Society, Papers of Magnetic Recording Seminar MR 75-29).
In order to enhance performances of magnetic photo-printing apparatuses, it is necessary to improve the characteristics 1 2 3 as described earlier. Therefore, the conventional problem is that it is not possible to obtain images with high resolution in the case when there is low resolution of a magnetic latent image such as that obtained by the magnetic recording medium that consists of an amorphous magnetic material and that is manufactured so as to have a greater film thickness.